Normal-form bisimilarity is a simple, easy-to-use behavioral
equivalence that relates terms in lambda-calculi by decomposing
their normal forms into bisimilar subterms. Besides, they allow for
powerful up-to techniques, such as bisimulation up to context, which
simplify bisimulation proofs even further. However, proving
soundness of these relations becomes complicated in the presence of
eta-expansion and usually relies on ad hoc proof methods which
depend on the language. In this talk, I will present a more systematic proof method to show
that an extensional normal-form bisimilarity along with its
corresponding bisimulation up to context are sound.

The talk is based on our recent paper (of the same title)
with Dariusz Biernacki and Sergueï Lenglet.